I'm trying to use this method to get my own horse textures in game. I've seen a couple people do it, and I really want to. Only thing is I can't find the textures, from what I hear they don't have actual names and are hard to look through. Which I don't mind doing I just need to know where to find them in OpenIV! 
Also if this guy's vid is outdated please let me know an alternative. Thanks

 

Or at the very least I want to swap textures on the horses

Go to nexusmods.com there's this person that is making retextured/reskinned horses and has already made alternate coats for MFT, Morgans, Turkomans, Nakotas, Mustangs, and Arabians. Look up bolju, that's their name.